Bug: Fix `pyrakoon.tx` concurrent command handling
Bug: Fix `pyrakoon.tx` concurrent command handling

When new commands are issued to a `pyrakoon.tx.ArakoonProtocol`,
handlers are pushed on a `deque`. Previously, these were retrieved from
the wrong side of the queue, which in turn results in a command reply
being parsed by the handler of another (possibly different) command, which
is obviously troublesome.

Incubaid/pyrakoon,Incubaid/pyrakoon